Detection Probabilities of Karst Invertebrates

Troglobites, or species restricted to caves, commonly have small ranges and high levels of endemism, making them extremely sensitive to relatively small-scale habitat alteration that results from urbanization. Protection of federally listed endangered troglobites in central Texas focuses on caves that are occupied by the species. The determination of occupancy is based on presence/absence surveys for those taxa. Under current U.S. Fish and Wildlife Service recommendations, three surveys are used as a standard to determine presence or absence, and certain environmental and seasonal conditions must be met. We used survey data from 23 caves on Camp Bullis Military Reservation, Bexar County, Texas that was collected from Fall 2003 to Spring 2007 (George Veni and Associates 2006) to create presence/absence matrices for three cave species, Batrisodes uncicornis, Chinquipellobunus madlae, and Rhadine exilis. Eleven environmental and seasonal covariates that have been suggested to affect detection probability (p) of these taxa were tested for fit to the detection data. B. uncicornis and R. exilis had weak likelihood scores for several covariates and thus were determined to have constant detection probabilities of 0.1226 and 0.1875. C. madlae was found to have a survey specific detection probability (average p = 0.2424), also not tied to any of the measured covariates. The calculated detection probabilities were used to simulate the number of surveys needed to have a 5% chance of not detecting the species if they were present at the site. The number of surveys needed ranged from 10 to 22. These analyses suggest that the recommended environmental and seasonal conditions for surveying these species are not related to their detectability, and that a far greater quantity of surveys than the recommended three should be performed to determine absence. The results also indicate that a significant proportion of the time cave species are not available to be surveyed, and we hypothesize that they retreat into humanly inaccessible cracks connected to the cave. If this is the case, in addition to the caves themselves these mesocavernous spaces should be a priority for conservation.